The Ca++ dependent exocytosis of catecholamine from adrenal medullary chromaffin cells is fundamental to the function of the adrenal medulla and has important cardiovascular and metabolic effects. It also serves as a model for exocytosis of catecholamines and other neurotransmitters and hormones from nerve terminals and cells. A major problem in the study of exocytosis has been the inability to alter the intracellular milieu of cells in a reproducible and convenient manner. In a preliminary study we have rendered leaky with digitonin the plasma membrane of bovine adrenal medullary cells in monolayer culture to extracellular Ca++ and soluble cytosolic proteins without significantly altering storage granule stability. Digitonin treatment exposes the intracellular processes involved in exocytosis to direct experimental manipulation. Exocytosis in digitonin-treated chromaffin cells requires micromolar Ca++ and is optimal in MgATP-containing solution. Digitonin-treated chromaffin cells will be used to study the intracellular processes involved in exocytosis. I will characterize and investigate the divalent ion and ATP dependencies, the role of calmoculin, and other soluble cytosolic proteins, the role of protein phosphorylation, and the role of osmotic forces in exocytosis.